Convert pixels to a physical print size, or millimetres to pixels, at a chosen PPI.
2,480px
209.97mm at 300 PPI
210mm
2,480px at 300 PPI
Pixels are divided by PPI to get inches, then multiplied by 25.4 to get millimetres. The reverse conversion follows the same steps.
A pixel has no fixed physical size without a PPI value. Keep the chosen resolution with the converted measurement.